1. Field of the Invention
The invention relates to a hot extrusion process for aluminum alloy, and more particularly to a method and mold group for producing seamless pipe sections of aluminum alloy using a horizontal single-cylinder extrusion press.
2. Description of the Related Art
Hot extrusion involves pushing a heated billet of metal through a die. The extruded product may be hollow or solid, and the cross section may vary from a simple round to a complicated shape. Direct extrusion, wherein the metal is forced under pressure through a die opening of the desired cross-sectional area and shape is the most common form of extrusion. The die is typically located in the end of a cylinder opposite a hydraulic ram.
The extrusion of products having a hollow cross section is complicated by the necessity to form the internal and external configuration of the part simultaneously. The portion of an extrusion die used to define the interior of a hollow extrusion is typically called a mandrel. The mandrel must be rigidly supported within the die to accurately define the interior configuration of the hollow extrusion.
The most common and least expensive type of extrusion equipment is the single-cylinder horizontal extrusion press. In such a press, a heated billet of metal is deposited in a cylinder where a ram forces the hot metal through an extrusion die at one end of the cylinder. Hollow products can be extruded on such a press by using a die in which the mandrel is supported within the die by a rigid connection or connections to an outer portion of the die. Such bridge, porthole and spider-type dies are widely used in the extrusion of hollow shapes from aluminum and aluminum alloy. In these types of dies (all modifications of the same basic construction), the metal billet is divided by the mandrel supports during extrusion and is forced back together as the metal xe2x80x9cchargexe2x80x9d passes through the remainder of the die and the die extrusion orifice. The metal welds under heat and pressure to form an apparently unitary hollow extruded product.
The two components of such a die are illustrated in FIG. 3 (mandrel supports not shown). The resulting extruded product, while appearing seamless in fact includes a weld seam or interruption of the molecular/crystalline structure of the metal wherever the material was forced to divide at a mandrel support. The presence of these imperfections at the molecular level can be confirmed by cutting across a pipe produced by such a method, polishing the cross-sectional surface to a high mirror finish, corroding the pipe in an alkali chemical agent heated to a temperature of 70-80xc2x0 C. for 10-15 minutes and then cleaning the pipe with water. The weld lines can then be clearly identified and appear in a configuration illustrated in FIG. 1.
A hollow extruded product having such structural imperfections does not have the structural integrity of a true seamless product. Typically, the seamed hollow product will fail along the weld lines or crystal interruptions when exposed to high internal pressure or large compressive loads. Therefore, such products are not widely used in industry in spite of the fact that there is great demand for hollow aluminum products in the manufacturing field. Seamless hollow tubular products, because of their strength and integrity are used to form tubular structural assemblies such as bicycle frames and are also able to withstand internal pressure and have improved corrosion resistance and are therefore in high demand for the aeronautics and food service industries.
Several methods are known in the art for producing seamless pipe sections of aluminum and aluminum alloy. FIG. 4 illustrates an apparatus for the centrifugal casting of seamless aluminum pipe sections. In this method, molten aluminum is poured into a mold that is rotated at high speed to form cast-aluminum seamless pipe sections of high integrity.
FIG. 5 is a functional block diagram of the equipment necessary for casting round aluminum rods. After the aluminum is melted in an oven 5-1, the melted aluminum passes into an insulated purifying oven 5-3 through a channel 5-2 where the aluminum is purified in an inert atmosphere. The purified molten aluminum then passes through an outlet 5-4 into a facility 5-5, 5-6 for producing solid aluminum rod by a water-cooled cooled continuous casting method. To form a seamless hollow tube, the solid rod must now be machined as is illustrated in FIG. 6.
FIG. 7 illustrates a more advanced method involving the continuous horizontal casting of pipe sections. In this process, after being melted in an oven 7-1, the molten aluminum flows into an insulated purifying oven 7-3 and then through a high-pressure water-cooled type graphite mold 7-4. The emerging tubular product is gripped by a tension-slip mechanism 7-5 that pulls the emerging product away from the graphite mold. While the tubular product is being pulled away from the mold, it is carefully cooled using high-pressure water. The process takes place from right to left in FIG. 7 with the molten aluminum emerging from the purifying oven 7-3 by force of gravity to flow through the graphite mold.
In a sophisticated extrusion process, illustrated in FIG. 8, two rams are used. After a first ram (sometimes called a floating mandrel) pierces the billet a second ram pressurizes the now hollow billet and forces the aluminum out of a die where the first ram/mandrel forms the internal configuration of the extruded product.
There are several disadvantages to the methods for producing seamless pipe sections of aluminum or aluminum alloy illustrated in FIGS. 4-8. The method of FIG. 4 requires the use of expensive centrifugal casting equipment and is limited to the production of pipe sections having a circular cross section. The method illustrated in FIG. 5 and FIG. 6 involves the expensive step of machining illustrated in FIG. 6 and results in excessive waste, i.e., material removed to form the bore of the tubing. The method of FIG. 5 and FIG. 6 is also limited to producing circular tubing. The method illustrated in FIG. 7 requires the use of expensive and not readily available continuous casting equipment.
Additionally, the methods illustrated in FIGS. 4-7 result in a cast-aluminum product in which the molecular structure, surface finish and dimensional tolerances require correction prior to use. Cast pipe sections are typically subjected to cold rolling as illustrated in FIGS. 9 and 10. These processes result in an increased molecular density on the surface of the tubing. It may also be necessary to use mechanical processes to reduce the wall thickness of the cast tubing.
Such cold working results in internal stresses in the crystal structure of the cast pipe sections. Therefore, the pipe sections must be heat treated to relieve these internal stresses as illustrated in FIG. 11 and avoid the hairline fractures of the tubing caused by such stresses. The heat treating process, while relieving the internal crystal stresses, typically results in pipe sections which are no longer straight or true. To produce straight pipe sections, each section must now be drawn through a stretching mill as illustrated in FIG. 12. The pipes are drawn through a tungsten carbide alloy mold with a straight stretching machine from which the seamless pipe sections emerge having a precision configuration which meets manufacturing tolerances. Following stretching, another annealing or heat-treating process must be done. The stretching process may be repeated with new tungsten carbide molds to further alter the configuration of the pipe sections.
Each step required to produce acceptable seamless pipe sections of aluminum or aluminum alloy adds to the complexity, expense and production losses of the process. While the method illustrated in FIG. 8 is relatively more advanced, the equipment is expensive, the process consumes a high amount of power and the technical requirements for achieving a tubular product having acceptable concentricity are high.
As a result, the prior art methods for producing seamless pipe sections of aluminum or aluminum alloy illustrated in FIGS. 4-12 result in an overall final product rate of not more than 70% (i.e. 30% waste). These factors result in very high production costs for acceptable seamless pipe sections of aluminum or aluminum alloy.
An extrusion die set in accordance with the present invention includes a charging die and a forming die configured to mate together and define an axially continuous, substantially tubular charge flow passage.
A preferred embodiment of the charging die includes a plurality of substantially identical charge distribution passages symmetrically arranged around an axis. Each charge distribution passage extends from an inlet to an angularly and radially offset outlet. Each charge distribution passage has a continuously variable internal configuration where the cross sectional area of the outlet is larger than the cross sectional area of the inlet. The charging die rigidly supports a mandrel at the mandrel end of an axially extending mixing extension having an opposed charging end. The mixing extension is supported at the charging end by webs of die material defined between said charge distribution passages. The mixing extension tapers from a maximum diameter adjacent the charge distribution passage outlets to a minimum diameter adjacent the integrally supported mandrel.
The forming die defines an axial opening configured to receive the mixing extension and mandrel. The internal configuration of the forming die cooperates with the external configuration of the mixing extension and mandrel to define a charge flow passage having several distinct portions. A mixing chamber in communication with the charge distribution outlets is defined by an outside surface and pressure surface within the forming die. An annular exit to the mixing chamber is defined between a pressure shoulder on the pressure surface and the outside or mixing surface of the mixing extension. A mixing space is defined beyond the annular exit in the extrusion direction by the inside surface of a tapering mixing cone inside the forming die and the mixing surface of the axially extending mixing extension. This mixing space has a continuously variable configuration in the extrusion direction due to the differing angular orientations of the inside surface and the mixing surface. The mixing space terminates in an extrusion orifice defined between the mandrel and a forming ring in the forming die.
A billet is forced through the die set from the charge distribution passage inlets and eventually through the extrusion orifice in an extrusion direction. A rotational component is imparted to the charge portions as they pass through the arcuate charge distribution passages into the mixing chamber. In the mixing chamber the charge portions are combined by being forced, under high pressure, through the restrictive annular exit. The now substantially combined extrusion charge is further blended in the continuously variable mixing space and finally extruded as a seamless hollow product.
An object of the invention is to provide a new and improved extrusion die set and method that permit the extrusion of seamless pipe sections of aluminum and aluminum alloy on a conventional horizontal single-cylinder extrusion press.
Another object of the invention is to provide an extrusion die set and method which permit the production of seamless pipe sections of aluminum and aluminum alloy not limited to round pipe cross sections.
A further object of the invention is to provide an extrusion die set for use in conjunction with a horizontal single-cylinder extrusion press that blends a divided billet so thoroughly during extrusion that the resulting extruded hollow product can be considered seamless.